Modern wireless communications systems operate in environments where signal quality to and from end-user devices is limited by interference coming from diverse sources. A wireless communication system may experience unexpected network interference originating from intentional and/or unintentional RF generating sources. These potential interference sources include other users served by the same or nearby base stations, industrial machinery, electronic test equipment radiating signals in the bands of interest, undesired mixing products generated by the wireless communication system, and illegal radio sources operating in the wrong frequency bands. The presence of these interference sources results in degraded system service and reduced wireless network capacity coverage as the intentional system signals suffer degradation due to these interferers.
Passive Inter-Modulation (PIM) may occur when one or more signals encounter a non-linear medium. For a single signal, PIM may generate new signals (hereinafter called PIM signals) with frequencies that are an integer multiple of the original frequencies of the signal. For two or more signals, the PIM may generate PIM signals having frequencies equal to sums of the frequencies of the two or more signals, differences of the frequencies of the two or more signals, and combinations thereof. As a result, the PIM signals may interfere with signals having frequencies other than the frequencies of the signals that caused the PIM signals to occur.
Non-linear media that generate PIM include ferromagnetic materials, corroded metal parts, and mis-joined connectors. For example, a rusted metal strut or a corroded electrical connector may generate PIM signals. Accordingly, the effects of time and exposure to the elements can cause PIM to occur and increase in items that when new and properly installed did not generate significant amounts of PIM.
Traditional methods for PIM detection involve manual on-site inspections which require intentionally disabling transmitting equipment and the generation of test signals. Manual inspections require turning off the revenue-generating network equipment and deploying personnel into the field, often at off-peak hours such as the middle of the night. Accordingly, manual inspections incur high costs and result in service interruptions.
Operators of communication system, especially wireless communications systems that operate over wide areas, may monitor their installations to detect new or increased levels of PIM that may cause unacceptable levels of interference. Conventional approaches to detecting PIM involve deploying technicians to base station sites to perform manual testing as described above, which is expensive, time consuming, and causes service interruptions. To reduce the cost of operating the communication system and improve its reliability, it would be advantageous to detect the existence of new or increased levels of PIM: 1) automatically, 2) without interfering with or interrupting the operation of the communication system, 3) without specialized signal generation equipment, and 4) without having technicians present at the installations where the interference caused by PIM is being received.
Interference caused by PIM affects frequencies that are different from frequencies that trigger the PIM interference, and in some cases a portion of the frequencies affected by PIM interference are used by a co-sited cell. In some installations, co-sited cells are operated by different operators which are generally market competitors, so that transmissions from one operator can cause interference that affects a different operator. Multiple types of interference affect frequencies and times that are used by different operators. In addition, non-PIM external interference can compromise the accuracy of PIM detection.